This invention relates generally to gas turbine engines, and specifically to the fan drive gear system of a turbofan engine.
Gas turbine engines provide efficient, reliable power sources for a wide range of aviation applications. In the particular area of fixed-wing aircraft, engine design has evolved substantially from the original turbojet concept to include a wide range of highly efficient and responsive turbofan designs.
Turbofan engines are built around an engine core formed by a compressor, a combustor and a turbine, which are arranged in flow series between an upstream inlet and a downstream exhaust. The turbine core may be coupled to a fan drive gear system that is connected to a fan, which accelerates flow from the inlet through a bypass duct arranged around the core. The core airflow is directed through the compressor, where it is compressed and then mixed with fuel in the combustor. The compressed air-fuel mixture is ignited to produce hot combustion gas, which drives the turbine and is exhausted downstream.
In two-spool turbofan engines, the turbine is divided into a high pressure turbine (HPT) section and a low pressure turbine (LPT) section. The HPT section is coupled to the high pressure compressor (HPC) section via a high pressure (HP) shaft, forming the high pressure spool, and the LPT section is coupled to the low pressure compressor (HPC) section and the fan via a low pressure (LP) shaft, forming the low pressure spool. The HPT, LPT, HPC, and LPC sections are each further divided into a number of stages, or alternating rows of blades and vanes. Individual blades and vanes are shaped as airfoils, and are configured to perform a number of functions including accelerating and turning the working fluid flow, compressing air in the compressor, and extracting energy from expanding combustion gas in the turbine.
The HPT and LPT spools are usually coaxially mounted, and rotate independently. The geared fan drive is used to provide independent control of the LPT/fan speed ratio, in order to increase engine efficiency, reduce noise, and improve turbofan performance. However, these advantages come at the cost of increased complexity required to connect the LPT to the fan drive gear system, which is a task that is complicated by the numerous components and systems that are tightly arranged within the engine.